Internal Revenue Service - IRS

What Does Internal Revenue Service - IRS Mean?
A United States government agency that is responsible for the collection and enforcement of taxes. The IRS was established in 1862 by President Lincoln and operates under the authority of the United States Department of the Treasury. It is primarily engaged in the collection of individual income taxes and employment taxes, but also handles corporate, gift, excise and estate taxes.

The IRS is sometimes referred to as the "tax man".

Investopedia explains Internal Revenue Service - IRS
The IRS is headquartered in Washington, D.C. It is an expansive organization that services the taxation of all Americans. In 2006, the IRS processed about 133 million personal income tax returns and almost six million corporate income tax returns, bringing in trillions of dollars of tax revenue.